Thats not really true
Poor dental health despite best efforts can be a sign of inflammation which can indicate a high risk of atherosclerosis as well rather than cavities cause heart disease
Plenty of people with rotten teeth and hearts like oxen on wheaties
Think Tibetans or Peruvians or Kenyan runners
Or old women with no teeth on the porch in rural America with snuff spittle on their chin
Not meaning to be argumentative
Poor dental hygiene may also contribute to heart disease.
And to the broader question asked, yes it does seem that the mass importation of people from the third-world results in our getting to deal with diseases that have been under control here for many decades.